Cheating in biology happens when one organism takes advantage of another organism to benefit itself without providing any benefit in return.
For example, imagine you have two students sitting next to each other during a test. One student, let's call him Bob, cheats off the other student, Sally, without giving her any help in return. Sally is nice and doesn't want Bob to get in trouble, so she shares her answers with him. Bob gets a better grade on the test without doing any work, but Sally doesn't get any benefit from helping him.
In the animal kingdom, some species have evolved cheater strategies to get food or resources from other organisms without doing any work themselves. For example, cuckoo birds lay their eggs in other bird's nests and let the other bird raise their chicks. The cuckoo bird benefits by not having to raise their own chicks, but the other bird has to work harder to feed the cuckoo chicks as well as their own.
Overall, cheating in biology involves one organism taking advantage of another organism without giving anything in return, which can have negative consequences for the organism being cheated.