Transactions are regarded as having happened when they are included in a new page in the list of all Bitcoin transactions.
Each page in the list is called a “block”. The list itself is a transaction journal called the Bitcoin “blockchain”.
Every Bitcoin wallet that works as Bitcoin’s inventor initially intended will talk to a few nearby wallets (or other “nodes” in the Bitcoin network) to find out about new unconfirmed transactions and new pages (blocks) in the transaction journal (blockchain). They each keep their own copy of the whole list from the beginning of Bitcoin in 2008. This process is called “synchronising”.
Some wallets may discard old blocks that are no longer of interest. This is called pruning. It isn’t really relevant to your situation but if I don’t mention it, someone may feel the need to point this out.
By communicating with others on the network, a wallet may discover that it lacks details of some recent transactions and tell you it is “synchronising”.
When you ask a wallet to pay someone. the wallet creates a transaction we call an unconfirmed transaction. This is like a draft transaction.
The unconfirmed transaction is only regarded as having really happened when it is added to a new page (block) of the transaction list (blockchain) by a miner. This is called a confirmation of the transaction.
Until a wallet is synchronised, it may not know about details of recent transactions and so may not yet know the outcome of recent transactions it has created.
