Transaction (What is a Transaction)
A transaction is a signed instruction to move value from one address to another, broadcast to the network for inclusion in a block.
In depth
Your wallet builds the transaction (who pays whom, how much, what fee), signs it with your private key and hands it to nearby nodes. Nodes validate it and keep it in a waiting area until a miner or validator includes it in a block. Once in a block it is part of the permanent ledger.
Why it matters
Every deposit and withdrawal you make on StableX is one of these under the hood.