When someone sends you bitcoin you receive a notification within a few seconds. You cannot however spend this money immediately. The computers in the Bitcoin network group transactions together and confirm them in 'blocks'. These are produced on average every ten minutes. Once a block is produced with your transaction in it the bitcoin is then spendable. You can see this on the 'Transactions' tab once the status icon is no longer empty.
Also, when you send a payment to someone, you normally get sent back some change. This is like giving someone a 'twenty' and getting 'ten' in return in a shop. When a transaction has definitely been seen by the network (ie confirmed or seen by two or more peers) then you can spend the change in a transaction.
The balance shown is the total amount of bitcoin you have in your wallet INCLUDING unconfirmed receipts and your change. The amount 'Spendable' is the amount in your wallet that is confirmed plus your change that has definitely been seen by the network. At any given moment, you can only spend up to the 'Spendable' amount.
Note: Older versions of MultiBit Classic used the expression 'available to spend' rather than 'spendable'. They mean exactly the same thing.