There are different meanings depending on context: 1 - is to believe of assume something is true/ take for granted - "I suppose the distance is 1km" or "It is supposed his death was an accident" 2 - to consider something as a possibility - "suppose we wait until tomorrow" 3 - to have an idea or opinion - "what do you suppose you'll do?" 4 - A requirement or expectation- "You are supposed to be working" or "the machine is supposed to make that noise" In your specific example it would be the 1st meaning. The speaker is saying that it is true, they can help but by saying "I suppose" it comes across as begrudging (agreeing to do something but not wanting to). If someone said "I suppose I COULD help" they are saying they are able to but might not.

Generally, the word "suppose" means there is "uncertainty". Like being "unsure" or "reluctant". EXAMPLE: "I suppose I can help" "I think I can help" or "I will help, if I need to"

I just checked and there is actually a third way to use suppose that I left out. The last way to use suppose is to express something that should or not be done. For example, instead of saying "I should be studying," you can say "I am (or in past tense, I was) supposed to be studying." The same works for negatives, like instead of saying, "I should not be here," try saying "I am not (or I was not) supposed to be here.

Suppose can be used primarily in two different ways. One is similar to I guess, or affirming something. For example "Did you like the food?" "I suppose." The other way is to introduce a hypothetical situation, almost similar to the phrase "think if..." So, for example, if you are introducing a hypothetical (hypothetical, if you aren't farmiliar, is an adjective used to describe something made up but possible) situation, you would say "Suppose this were to happen". I hope that explanation made sense :)