What does equilibrium in terms of solutions mean, & what is cytosol concentration in potatoes (not the number)

Equilibrium doesn't actually mean a thing to a solution by itself.  However, if you're dealing with solution A on one side of a membrane (like a cell's membrane), and solution B on the other side of a membrane (like inside of a cell), then equilibrium means that the solutions are "balanced".  

In some ways, you could think of it as the solutions being the same.  This isn't technically true, but it gets the basic idea.  The solutions on either side of the membrane are balanced so that neither one has a charge (positive or negative), both have fairly equivalent salts in them, etc.  

As far as a cytosolic concentration in potatoes, it's hard to know what you mean without more details.  However, the cytosol is basically the free space inside of cells.

Cells are filled with all sorts of things.  There are a lot of organelles (like organs in a human, just on a smaller scale), but everything floats around inside of the cytosol.  You can imagine it as a swimming pool filled with bath toys--the bath toys are organelles; the water is the cytsol; the walls of the pool are the cell membrane.

What happens when you mix in a bunch of swimmers?  You could imagine swimmers as the salts and other things in the cytosol.  The cytosolic concentration is the number of swimmers divided by the area of the pool.  The outside concentration is the number of swimmers divided by the area around the pool.

When there are equal concentrations of swimmers standing outside of the pool and inside of the pool, the pool and the area around the pool are in "equilibrium".  Remember to consider area here--if there is twice as much space in the pool as outside of the pool, we don't hit equilibrium until twice as many swimmers are inside the pool as outside of the pool.

The cytosolic concentration would be the number of swimmers in the pool divided by the volume of the pool.  Keep in mind the "bath toys" (organelles) that I mentioned earlier.  These take up space, too!  The total volume of the pool doesn't actually count for concentration--it's the total volume of the pool MINUS the bath toys (organelles).

Lemonade is a good example here.  The concentration of lemon juice (tastiness) is the same whether you have half a cup, or a full cup with half a cup of lemonade mixed with half a cup of ice cubes.  The ice cubes increase the total volume (and make it a full cup), but the concentration of flavor stays the same.