Initial block download (IBD) refers to when nodes synchronize themselves to the network by downloading new blocks. This will happen when a node is far behind the tip of the best blockchain. In the process of IBD, a node does not accept incoming transactions nor request mempool transactions.
If you are trying to set up a new node following the instructions below, you will go through the IBD process at the first run, and it may take a considerable amount of time since a new node has to download the entire blockchain (which is roughly 140 now). During the download, there could be a high usage for the network and CPU (since the node has to verify the blocks downloaded), and the client will take up an increasing amount of storage space(reduce storage provides more details on reducing storage).
Before the node finishes IBD, you will not be able to see a new transaction related to your account until the client has caught up to the block containing that transaction. So your wallet may not count new payments/spendings into the balance.
If you are using Bitcoin Core GUI, you can monitor the progress of IBD in the status bar(left bottom corner).
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