A very small proportion of the whole.
From the Bible, Isaiah 40:15 (King James Version):
“Behold, the nations are as a drop of a bucket, and are counted as the small dust of the balance: behold, he taketh up the isles as a very little thing.”
‘A drop in the bucket’ is the predecessor of ‘a drop in the ocean’, which means the same thing, and is first found in a piece from The Edinburgh Weekly Journal, July 1802:
“The votes for the appointment of Bonaparte to be Chief Consul for life are like a drop in the ocean compared with the aggregate of the population of France.”