It means that you won't be able to resize the given column in the editor, unless you use the equalize function or remove the lock.
It's useful when it's really important to keep a certain column at a fixed width while editing.
When the lock is on, the proportions are locked, not the exact pixel size. So if you move the whole multi-column in to a different, narrower location inside the email, even the locked column will resize proportionally.
Here is an example:
Lock width switch off
While I'm editing the left column, increasing the padding of the left columns (the image), the column proportions will change when the Lock width of the Multicolumn is off.
The percentage was 20.08 on the left and 79.92% on the right column.
I changed the padding of the left column and here is the column percentage now.
27.73% on the left and 72.27% on the right column.
Lock width switch on
While the Lock width switch is on, I changed the padding again in the same way, and the column percentage hasn't changed, it stayed 20.08% on the left and 79.92% on the right column.