Anchors are points that go with a certain glyph and to which other glyphs' anchors may attach. Consider the e with macron and breve in this example:

The e has an anchor above the glyph, to which the anchor below the macron is attached. The macron glyph has not only an anchor under, but also an anchor above, which attaches to the anchor under the breve glyph. In this way, many glyphs may automatically positioned relative to each other without having to specify positions for each combination separately.

OTComp expects to find anchors in the format <x,y>. Thus, <50, 170> is an anchor with x coordinate 50 and y coordinate 170. By adding a colon before the first coordinate, the anchor is mapped to a contour point, which may be explicitly positioned using glyph instructions (for example by using TTIComp). Thus, <: 50, 170> will produce a contour point anchor at (50,170). If there is no contour point with that coordinate yet, it is added. OTComp is fairly smart about adding points in composite glyphs, so you do not have to worry about that.