We call 'wet', the return from an FX processor. 'Dry' is the input signal to the effect. So if you turn 'wet' down and 'dry' up, the signal will pass through the effects unit unchanged.
The reason many effects units have a dry/wet setting is so you can either use them as insert effects, or send effects. An insert effect is one where all of a channel's signal passes through the effect and comes out the other side (output) - mixers have an input/output in a single jack for this use. Send effects on the other hand are those where you want to be able to hear the direct sound + the effect - mixers usually have potentiometers named 'Aux(illiary) sends' for this use (among others). There are also auxiliary return channels on most mixers where you can return the sound from the effects unit.
So if you used this delay effect as an insert you would usually turn the dry level all the way up and add as much of the effect as you like using the 'wet' control. But if you used it as a send effect, you would usually turn the dry level all the way down - something I kept forgetting in my early days - to avoid doubling your level. Then turn the wet level all the way up and control the final level of the effect from the aux return.
In the diagram bellow, these are the two signals mixed on the output mixer (+).
The picture above is the diagram of a simple, mono delay unit. The input signal going directly to the output is your 'dry' signal. It also goes into the delay line and some of the delayed signal is fed back and added (=mixed) with the input to be delayed again and again and again...
If you set the feedback level to 100%, you will hear continuous repetitions until you turn it back down. If you turn feedback all the way down, you will only hear 1 repetition.
There is also a 'color' slider which I suspect controls the cutoff frequency of a filter. This filter is commonly placed in the feedback loop (where that triangle is) to make each repetition sound a bit different. If it's a low pass filter, the signal will sound duller with each pass through the feedback loop. So if we assume this filter is an LPF, even with 100% feedback, the repetitions would not last forever in some cases.
It is worth mentioning that by returning the delay output on a normal channel instead of an aux return, you can have an external feedback loop with any (insert) effect you wish inside it including the very useful EQ - just send it back the the delay using the aux send on the return channel. But beware. Anything including the insert effects that increases the level in the feedback loop can make your levels explode, as each repetition will be louder instead of quieter than the previous one. Still, this technique is very useful in making more complex repetitions like pitch-shifting them for example.