You try to send some bitcoin but when you press the 'Send' button you get the error 'Insufficient funds' appearing in a dialog box.
In your wallet you can normally only spend bitcoin that have confirmed. This is shown graphically in the 'Transactions' tab when the status icon is no longer empty. If you have unconfirmed transactions you get an 'Available to spend' balance in the MultiBit Classic header. This shows the amount in your wallet that is available to spend. See What does 'available to spend mean ?' for more details.
Also, you need to take into account the fee that is added to your transaction. The fee varies according to the size of the transaction but is typically 0.0001 BTC. For instance, if you have 10 BTC in your wallet and the fee is 0.0001 BTC then you can only spend a maximum of 10 - 0.0001 = 9.9999 BTC.