You often hear the phrase, "go with the grain". This can be confusing. A woodworker needs to identify the rays and growth direction of a board. Grain is often referred to as the visual you see on the surface of the board, this is created by the annual growth rings. Sometimes the rays match the growth rings and sometimes they go opposite. A good way to figure this out is to take a utility or other sharp knife and try to slice a small 45 degree chamfer on the edge of a board. If you go one direction, the knife will slice the chamfer nicely. Turn the knife and go the other way on that same edge and it will dig in or the wood will split out. When you run through the planer you want to identify this direction and have the planer knives cutting, "with the grain".
I have a picture of a piece of red oak, similar to ash. What you look for in red oak is those tiny brown slashes the pencil tip is pointing to. In this picture, the "grain" runs opposite the direction of the growth rings. Every species has such direction characteristics that you need to be able to identify but they aren't all little brown slashes. Not all boards have continuous same grain direction throughout the length. It can change directions on the same board.
If you get tear out, turn the board end for end on the next pass. In most cases, the grain runs opposite on the other face. You have to turn boards end for end when planing both faces or you will go against the grain on one of them. Many pre surfaced boards can have this problem because they are run on double sided planers. Sharp knives and slower feed can help with tear out.
There are figured species, like birds eye maple or curly cherry, to name two, where the grain goes around in a circular pattern or abruptly changes direction. When you hear about York angle frogs on a smoothing plane, the blade angle has been raised to engage the wood at a steeper angle of attack. This is one way to deal with the tear out you would get with this type of lumber if you used an ordinary smoother at the normal frog angle. Unless you are using an abrasive planer or a wide belt sander, it can be difficult to completely eliminate tear out, especially when the grain gets a bit wild.
Both oak and ash, as well as other species, will often splinter on the edges. If you were to sand an edge to round it over a little and you go back and forth, you'll likely catch and maybe get a nasty splinter. You can go in the correct direction but you don't want to go back and forth. If you mill in the right direction, ash is usually pretty nice to work. Pick boards up on the end and sight down the edges to see grain direction. It will take some practice to see it but it makes a huge difference in the quality of the surfaces.