Evolution is caused by genetic mutations that become favorable for survival.
For example if a bird has a mutation that gives it a longer beak and it lives in an environment in which a longer beak lets the bird catch more food- the longer beak becomes a favorable trait.
Now the bird is able to survive “better” thus it lives longer(due to its ability to catch more food). Because of this it reproduces more and passes the trait on to its offspring.
Overtime this mutation takes over the population of birds in the area.