New to all of this. Which ranges of inner tube would fit in a 29 X 1.4 SLICK MOUNTAIN BIKE TYRE WILD RUN'R? Should I also worry about valve size? If possible, can I have a good source for learning what those numbers mean?
Bike tire sizes are a confusing topic. This page is a bit dated, but still worth bookmarking.
There are two valve types: Schrader and Presta. Presta is skinnier and will fit loosely in a Schrader opening; you can get adapters to shim up the difference (technically there's a third type, Dunlop, but I've never actually seen a Dunlop valve).
29" is mountain-bike terminology for what roadies would call 700C; both have a bead-seat diameter of 622 mm. So you can use a tube marketed for road bikes, although a 1.4"-wide tire (about 35 mm) is at the outer edge of what you'd find on road bikes.
In general, you have some latitude when fitting innertubes. As long as they're not bunched up or stretched taut inside the tire, they're fine.
This question is probably going to get closed as a duplicate, but to answer your valve point:
Yes, valve size matters. If you have a Schrader/AV/car valve then its wider than the other formats of Presta/Dunlop/Woods valves. Those last three are interchangeable with a 6.5mm hole in the rim, and the car valve needs an 8.5mm hole and won't fit through the smaller hole.
It is possible to put the other valves through a larger schrader hole, but they tend to puncture because of the lack of support at the shoulder. You can get a grommet to take up the slack, or some valves have a retainer nut with a shoulder on one side to help, but in my experience you really want the right valve for the hole, and that suits your pump.