Ties and struts
Tension members in trusses are called ties and these are members which are being stretched. It is an industry convention that the arrows are shown pulling in on themselves. This is in contrast to the tension in a beam in which the tension forces pull outwards from the beam as shown in the bottom diagram.
Compression members in trusses are called struts and these are members which are being shortened. The industry convention shows arrows which are pushing outwards. Compare this to the compression in a beam in which the forces push inwards as shown in the bottom diagram.
Note that the arrow convention for labelling struts and ties in trusses is opposite to the convention used for tension and compression.
You may find it helpful to think of tension and ties together as a memory aid as these represent the same force.