Mining is the process of specialized computers verifying all the transactions on the Bitcoin base layer.
Say I am a customer at Bank of America and you're a customer at Wells Fargo and I use Venmo to send you money. What happens? Employees, time, and electricity is spent on updating my account balance at Bank of America and your balance at Wells Fargo.
The Bitcoin network turns that whole concept on its head and says how about instead of Bank of America and Wells Fargo having separate ledgers that they each maintain, how about we all work off one ledger and how about we each keep a copy of it so that we all know that everyone's balances are being kept fairly.
Mining is the process by which specialized computers do this for the Bitcoin network, they ensure the security of everyone's address balances and make sure that no one is breaking the rules.
Transactions between users are grouped together and a set of them are verified and committed to the network every 10 minutes in blocks. Miners get paid a portion of the transaction fees to secure each block.