In general people use the effects loop to insert certain effects after the preamp stage but before the power amp stage in their amplifier. The preamp stage is where the EQ is applied and this stage colors the sound the most. The preamp is also where you get some of the overdrive when you turn up the gain. The power amp is usually more transparent and mostly just amplifies the signal coming from the preamp enough so it can push the speaker.
There are many types of pedals, and some work better before the preamp (these go in between your guitar and the amp's input) and other pedals work better after the preamp (these go in the effects loop). Distortion/gain pedals will almost always go before the preamp, whereas time-based effects like delay and reverb are usually in the effects loop if there is one available. Modulation pedals like chorus or flanger can go anywhere, but do some experimenting to figure out what sounds best in your setup. Some people prefer modulation pedals to go before their distortion pedals, so in that case they would all go into the front of the amp. If you want the modulation pedals to go after distortion, you can do it either into the amp's input or in the effects loop, each with different results.
These are just guidelines but you should try different combinations and orders of pedals to see what you like best. Basically, any effects that you want to be "colored" by the preamp can go into the input, while effects that work best on a cleaner signal can go in the effects loop.
All that being said, for me I don't really worry about effects loops on bass amps. It's useful when playing guitar to be able to split the signals and really tweak the effects, but on bass I'm only ever using a couple pedals at a time so they usually work just as well in front of the amp. I also don't use the amp's built in distortion/overdrive on bass amps, I generally just play with a fuzz pedal and maybe some delay and/or chorus and they work out just fine in front of the amp. In fact I don't even use effects loops much for guitar either, I usually like to run everything into the front of the amp. Everyone has a different opinions on effects loops, like I said I rarely use them but some people can't live without them. Neither way is better- effects loop or not- it's just a personal preference. If you are still experimenting with effects and finding out what you like, I wouldn't worry too much about the effects loop. You can get a lot of variety out of a couple pedals into the front of the amp.
Edit (a bit more on bass effects in general):
When I play bass I don't like using a lot of effects because the bass has a pretty important role to play in the low frequency realm and effects often either cut the lows or just muddy up the sound. I do have a Big Muff fuzz pedal and I like it with bass, but it cuts out a lot of the low end of the signal so it's not something I would use for a whole song- maybe just for the chorus or a bass solo. I rarely use any pedals other than fuzz/distortion on bass, but if I had to pick a couple I would probably say a delay pedal could be useful as well as one modulation pedal- probably either a chorus or phaser. Distortion + chorus can make a great sound or use a phaser/flanger for some movement. Distortion + delay is another great combo for solos. If you don't have any pedals yet just start with a good fuzz or distortion pedal and see how you like it. When it comes to bass, effects aren't for everyone.
