Yes, it's is simply the short form of it is: It's raining and it is raining mean the same thing.
One point that may affect your choice: in formal writing, some readers treat contractions as too casual, so the full form is safer there. The paired examples in the next section show how this plays out, with it's suiting a casual message and it is suiting a formal one.
Paired examples
It's ready when you are. In a casual chat, the contraction keeps the message light. It is ready when you are. Same words, but the full form adds weight, as if you want the reader to slow down.
It's on the second floor. A quick text reads naturally with the contraction. It is on the second floor. This works too, especially in instructions where each sentence should sound complete and deliberate.
I checked, and it's fixed. Reporting back to a colleague, the contraction sounds conversational. I checked, and it is fixed. Here the full form stresses that the problem is genuinely solved.
Neither version is wrong in most everyday writing. The choice is mostly about tone: the full form is the safer default, the contraction the friendlier one.
When a reader might object
Try swapping in it is and reading the sentence aloud. If it sounds stiff for the setting, use it's. If the reader is new, distant, or evaluating you, the full form is safer.
Which version would you actually send? Picture a busy manager you text politely to confirm a time. Would you write it's or it is?
When your message is going to someone you address politely but not formally, would you write it's or it is to confirm a time?
I'll check whether it's free. Casual and quick, right for a chat with a teammate. The contraction matches how the two of you already write.
I'll check whether it is free. This also works. It reads a touch more deliberate, which suits a client or a first exchange. Neither choice breaks the sentence.
You are confirming a meeting with a new client. Full form suits the setting: It is confirmed for Tuesday at 10. Some readers judge contractions too casual there, and a few will mark them down, so the longer version avoids the risk entirely with no real cost to tone.