Most are there due to BASIC language progeny:
Let x = 1 - assignment, same as x = 1
Pos(), Tab() - positioning inside PRINT
Dim a[10] - pre-sizing an array (from DIMension), same as a = createObject("roArray", 10, true); both are almost never needed because of auto-sizing and {} literal notation; rare `dim multiDimensional[10][20]` has advantage of being short
Next - same as "end for", but shorter
Step - member of the "counting troupe": FOR i = 1 TO 10 STEP 2
i believe all are mentioned in TFM, curtly as it may be.