Most kinds of plants have two sets of chromosomes in their vegetative cells and are said to be diploid, i.e. each chromosome has a partner that contains the same, or similar, genetic information. By contrast, mosses and other bryophytes have only a single set of chromosomes and so are haploid (i.e. each chromosome exists in a unique copy within the cell). There are periods in the moss life cycle when they do have a double set of paired chromosomes, but this happens only during the sporophyte stage.
The life of a moss starts from a haploid spore. The spore germinates to produce a protonema (pl. protonemata), which is either a mass of thread-like filaments or thalloid (flat and thallus-like). Moss protonemata typically look like a thin green felt, and may grow on damp soil, tree bark, rocks, concrete, or almost any other reasonably stable surface. This is a transitory stage in the life of a moss, but from the protonema grows the gametophore ("gamete-bearer") that is structurally differentiated into stems and leaves. A single mat of protonemata may develop several gametophore shoots, resulting in a clump of moss.
The evolutionary thinking of significance based on this topic is not known by expertise for sure. All that's potentially know is that the relationships between the two have a consistent genetic base and relations that both Protonema and Moss-Alga-e will contain similar chromatic genes as well as similar DNA contained in the nucleus and Chromatin of the Chromosomes inside of the Similar Protonema and Moss cells.