I've been told multiple times that it has no definite meaning and also that in general it is used with more abstract actions and ideas. Why was this suffix created, and is there any way to assimilate the meaning of a particular word using it?
Zamenhof created the Esperanto affixes so as to reduce the burden of learning vocabulary: It is easier to memorize a word that is formed through an affix based on an already known word than to memorize a completely new word root.
Zamenhof originally came up with 36 affixes that correspond to 36 different ways in which the meanings of two words can be related, and he used them to greatly reduce the number of word roots that need to be learned separately. But he also realized that there are many words that have a meaning closely related to the meaning of another word, but which did not fit well any of the affixes that he had created. Instead of creating many more affixes for special meaning that are only very rarely needed, he decided to use a joker suffix for these cases. This joker suffix is -um-.
Even though the meaning of a word with -um- is not predictable in a way that it is for other affixes, this suffix is still very useful, as it is still easier to memorize a word with -um- than to learn a completely new word root, and it is easier to guess the meaning of a word with -um- from context than in the meaning of a new word root.
Note that the joker suffix -um- is similar in nature to the joker preposition je, which doesn't have a definite meaning either, but is used when none of the other prepositions makes sense.
Given its nature of a joker suffix, in general you cannot know the meaning of a word using it without having learned it. Nevertheless, there are some meanings of -um- that come up in several words with -um-. PMEG lists five such "regularized" meanings of -um-:
- To do a certain thing with what stands before -um-, e.g. brakumi (to do a certain thing with your arms, i.e. 'to hug'), and palpebrumi (to do a certain tin with your eye lids, i.e. 'to wink').
- To provide with what stands before -um-, e.g. aerumi (to provide with air, i.e. 'to air' in the sense of 'to let air in') and sunumi (to provide with sun, i.e. 'to sunbathe').
- To execute a person in a certain way, e.g. krucumi ('to crucify') and ŝtonumi ('to stone to death')
- A part of a clothing at the body part indicated before -um-, e.g. kolumo (the part of a clothing at the neck, i.e. 'a collar'), and plandumo (the part of a clothing at the sole of a foot, i.e. 'a sole of a shoe').
- Number system, e.g. duuma ('binary') and dekuma ('decimal').
However, many words with -um- don't fit any of these five categories, e.g. malvarmumo ('a cold'), and plenumi ('to fulfill').
Unless you are a very experienced Esperanto speaker, you should not make up new words with -um- that don't fit any of the above five categories.
"Um" is generally used when there is no other root or suffix which matches the definition and the word is too specific to create a new root or suffix. For instance:
aerumi = to expose to the air
Check here for a list of the most common -um words.