Bloom refers to a phenomenon which occurs on a varnished surface of a painting if kept in damp conditions. This results in dull and cloudy areas on top, or sometimes below the varnish. Blooming is basically a result of moisture building up on the painting.
It if occurs on top of the varnished surface, you may be able to remove it with a clean and slightly moist rag. If it is below the surface, you would need to remove the layer of varnish first and attempt to clear up the bloom. If funds are not an issue, then you should seek a professional to address any blooming if you are not familiar with it yourself.
To avoid blooming, you need to be careful when applying a varnish on your paintings. Do not varnish your paintings on dreary and humid days, as you may end up trapping moisture under the varnish.