When you connect the terminals of a voltage source to each other, you create a short circuit. This means a high current flow. To limit the current flow, you can use a resistor. The symbol of a resistor is:

Voltage, current and resistance are related to each other as follows:

R = V/I

V is the voltage across the resistor [unit: volts, or V]; I is the current in the resistor [unit: amperes, or A]; R is the resistance [unit: ohms, or Ω].

Example: Imagine you connect a 1000Ω (or 1kΩ) resistor to a 9V battery. In that case, the current in the resistor (and in the battery of course!) will be: I = V/R = 9V / 1000Ω = 9mA (milli-amps).

You can't buy resistors of any value. You can choose from a series of resistors, e.g. the E12 series. The E12 series has the following values: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82. If you want other values, you may select one from another (more expensive) series, or create one by connecting multiple resistors in series or parallel.