"X" in stock solutions
1x is the final working concentration of the solution (it could be anything depending on the type of the solution). Stock solutions are made at a higher concentration; if it is 10 times more concentrated than the working solution then it is 10x. 


  Why 50x TAE but 10x TBE?


The salts in TBE precipitate at higher concentrations. In other words, the salts in TBE have lower solubility compared to those of TAE. Therefore stock solutions of TBE are usually 10x or even 5x.