Rephrase 1
A suntan is when your skin gets darker from sunshine. The darker color helps guard your skin a little bit from getting sunburned.
Rephrase 2
Your skin can turn a darker shade after sunny days. That darker shade works a bit like sunscreen from the sun itself, helping prevent sunburn.
Rephrase 3
After time in the sun, the skin may become browner. This browner color helps protect the skin some from getting too red or burned by the sun.