An atomic spectrum is a spectrum which was shined through or came from a material and contains structures that are qualities of the elements existent in the material. For instance, if you examine the Sun's spectrum, you can find out what elements are existent in the Sun.
When atoms are excited, they produce light of particular wavelengths which signifies different colors. The emitted light can be seen as a sequence of colored lines with dark spaces in between; this series of colored lines is known as atomic spectra a line. Each element produces a unique set of spectral lines.