Semantics is the branch of linguistics having to do with what words mean. Words, of course, only have meaning because we as human beings give them meaning. Therefore, we humans can be said to "invest" things like words with meaning. Taken a step further, we can invest things other than words with meaning. We can, for example, invest a car with meaning. We can look at the car and say that it symbolizes something (a luxury car symbolizes wealth, a hybrid symbolizes "greenness," etc). That is part of semantic investment. It is the process of giving meaning or significance (Greek sēmantikos; being significant) to things that could have other or less significant possible meanings.
Depending on the exact line in the book you read, it seems to be referring to the idea of people ascribing significance and meaning to the human experience. I cannot tell for sure without seeing the quote, but this is a likely meaning for the phrase.