We use "select for" when talking about breeding animals or plants or about evolution and natural selection.
select for + a characteristic or trait
Some types of dogs have been deliberately bred over centuries to, for example, be good at guarding sheep. The traits that make dogs good at guarding sheep have been selected for. In other words, humans have picked the animals which best display those traits and bred them together.
The natural process of evolution has also selected for certain traits. Biological characteristics in animals have become more common (selected for) or less common (selected against) over millions of years of natural selection.
I suppose the difference between 'select for' in this sense and 'select' is that it is not 100% guaranteed process. You can only select for certain characteristic (favour them, try to enhance them), you can't guarantee the outcome. If you 'select' something it implies you have control over the outcome - I want THAT one - but 'select for' implies you are setting up the best conditions for your favoured outcome but it might take a long time and a lot of stops and starts to get there.
Hope that makes some kind of sense, Terry.