resistors in series
Resistors in series are joined end to end in a single file, so there is exactly one path for current to follow. Think of several narrow sections of pipe lined up one after another: the water has no choice but to pass through every one, and each section adds its own restriction. Because there is only one path, the very same current flows through every resistor in the chain.
Their resistances simply add up: R_total = R1 + R2 + R3 and so on. A 100 ohm and a 220 ohm in series make 320 ohm. The total is always larger than any single resistor, because you keep adding restriction. The supply voltage divides among them in proportion to their sizes, and those individual drops always sum back to the supply voltage, which is just Kirchhoff's voltage law at work. For example, 5 V across a 100 ohm plus 400 ohm series pair drives 10 mA, and the drops are 1 V and 4 V, adding back to 5 V.
Series strings show up everywhere: making a value you do not stock by adding two you do have, dropping a voltage on purpose, or limiting current. The key mental picture, and the most common beginner mistake to avoid, is that series resistors share the same current but not the same voltage, while parallel resistors are the exact opposite.
You need 1.3 kohm but only have 1 kohm and 330 ohm in your kit. Wire them in series: 1000 + 330 = 1330 ohm, close enough. The same current flows through both, and the 9 V supply splits as about 6.77 V and 2.23 V across them.
Series resistances add, letting you build values you do not have in stock.
Series resistors share the same current, not the same voltage. Mixing this up with the parallel case is the classic beginner slip.