The most likely answer to that question is a stye.  The medical term for a stye is a hordeolum.

Almost all of us will get at least one hordeolum in our lifetime.  Some of us will get them regularly.

A stye is not really an infection, it is just a plugged meibomian gland.  A meibomian gland is an oil gland and each of our eyelashes has one of these oil glands next to it.  These oil glands are a great place for bacteria to live because it is warm, dark, and the oil is a good food source.  There are always a tiny amount of bacteria in those glands.  However, if we are fatigued, dehydrated, under immune stress, or FOR NO REASON AT ALL, those bacteria can become too plentiful.  The excess bacteria cause the oil gland to become clogged and swollen.  The oil in the gland becomes like butter instead of oil and the result is a stye! 

The most important part of the treatment for a stye is warm compresses.  The warm compress really just melts the ‘butter’ back into oil.  To supplement the warm compresses, we often add lid scrubs to clean the eyelash margins after the warm compresses and then complete the treatment with some antibiotic ointment to the lid margins.  The antibiotic ointment is the least important part of the treatment.

The stye may have a pimple like appearance and it can be tempting to pick at it or pop it.  DON’T do that.  Popping it can cause the gland to break inside your lid and those few bacteria can cause a real infection in your lid that require oral antibiotics to treat.

Very rarely, a hordeolum does not resolve with this treatment and surgical removal is required.  This usually only happens when treatment is not initiated quickly enough or the warm compresses are not applied frequently enough or for long enough.