Sentence has two main meanings:

1. In grammar, a sentence is a complete thought expressed in words. It typically has a subject (who or what the sentence is about) and a predicate (what the subject does or is). “The dog barked” is a simple sentence. “Although it was raining, we played soccer anyway” is a more complex sentence. When you write, every sentence should make sense on its own and usually starts with a capital letter and ends with punctuation like a period, question mark, or exclamation point.
2. In law, a sentence is the punishment a judge gives to someone convicted of a crime. After a trial finds someone guilty, the judge sentences them, which might mean prison time, community service, or a fine. You might read that a thief “received a sentence of two years in prison.”

When teachers talk about “sentence structure,” they mean the first definition. When news reports mention someone's “prison sentence,” they mean the second. Though these meanings seem different, they both involve making a complete, official statement: either completing a thought in writing or declaring a legal consequence.