Put two resistors end to end — a single loop, no branches — and you have a series circuit. The same current threads through both, like one line of people passing through two doorways. Each doorway slows the line, so the resistances simply stack: $R_t = R_1 + R_2$ (read aloud: R-total equals R-one plus R-two). The little numbers are only name tags: $R_1$ and $R_2$ are the two resistors in ohms, numbered in the order you meet them, and $R_t$ — t for total — is the single resistance that could stand in for the pair.

The working order never changes: total the resistance FIRST, then let Ohm's law read the whole loop — $I = \dfrac{V}{R_t}$, the battery's push over everything it must push through. Two small jobs, in that order, and every series problem in this room opens the same way.