A species cannot afford to accumulate deleterious mutations in the genomes of its individuals. Eventually the number of "bad" mutations will reach a level where most genes have multiple "bad" alleles and it becomes impossible to produce offspring.
This phenomenon is referred to as genetic load. It means that species can only survive if the genetic load is below some minimum value. A good rule of thumb is that there can't be more than 0.1 deleterious mutations per individual per generation but in actual populations this value can be a bit higher. [UPDATE: This should be one (1) deleterious mutation per generation.]
How do you reconcile this with the known mutation rate in humans? If there are, on average, 130 mutations per individual per generation, then hardly any of these can be deleterious if the species is to survive.
This is one of the arguments in favor of Neutral Theory. Most mutations are neither deleterious nor beneficial. They are simply neutral with respect to natural selection.
Let's think about a typical protein-encoding gene.1 The coding region is about 2,000 base pairs in length and consist of 666 codons. More than half these codons can be mutated to a new codon encoding a different amino acid without severe effects on the function of the protein.2 These are called amino acid substitutions. Of the "essential" codons, many can tolerate mutations that create synonymous codons. Putting these facts together suggests that only about 20% of mutations to protein encoding regions are detrimental. The rest are effectively neutral.
This partially explains why we can tolerate 130 mutations per individual per generation. If only 20% were detrimental then the genetic load is reduced to about 26 mutations per generation.
That's still unacceptably high. It leads to the idea that a large percentage of our genome must be unaffected by mutations. In other words, genes represent only a small percentage of our genome and mutations can freely accumulate in the rest without detrimental consequences.
In order to bring the genetic load down to acceptable levels, the number of genes has to be less than 40,000 according to the arguments made in the 1960s. We now know that we have only 20,000 genes. Most of them encode proteins and the coding regions of those genes make up about 40,000,000 bp or about 1.3% of our genome [Junk in Your Genome: Protein-Encoding Genes].
Recall that only 20% of mutations in coding regions are likely to be detrimental. That means that the effective target size for detrimental mutations is about 20% x 1.3% = 0.26% of our genome. Out of 130 mutations, only 0.3 per individual per generation will be detrimental.3
Since we are diploid organisms, the 130 mutations in the zygote are spread out over two copies of our genome but almost all of them will be in the chromosomes coming from the father. Every zygote inherits one complete set of chromosomes with hardly any mutations while the other set has less than one detrimental mutation.
Because a large percentage of gene mutations are neutral, and because most of our genome is junk, we can easily tolerate 130 mutations per individual per generation without going extinct.
Creationists will never understand this because: (a) they believe that modern evolutionary theory is all about "Darwinism" and Darwinian evolution doesn't recognize neutral mutations and random genetic drift, and (b) they can't admit to junk DNA because that's the opposite of what intelligent design would look like.
1. Similar arguments apply to genes that make functional RNAs and not proteins.
2. Over the course of several billion years of evolution it is unusual to see more than 30% sequence similarity between homologous genes. I realize that this is a somewhat circular argument but it's still a good one.
3. There are lots of other regions of the genome where mutations can be detrimental. I don't mean to imply that only protein encoding regions can be affected by mutations. Collectively, these other regions don't make up more than a few percent of our genome and they can tolerate many mutations [Genomes & Junk DNA]