The alb is the tunica alba, the white tunic, and like the cassock it is a surviving piece of ordinary late antique clothing. What made it liturgical was not a decision to invent a vestment but the accumulation of meaning around a colour.
That meaning runs through the whole life of a Christian rather than through the sanctuary alone. The newly baptised in the early church were clothed in white after coming up from the water, and the white garment is still given at baptism in most Western rites with words about putting on Christ. The pall laid over a coffin at a funeral is white and is explicitly identified in the rite with the garment given at baptism, so the same symbol closes the circle. Behind both stands the vision in Revelation of a multitude in white robes.
Around the alb sit two smaller items. The amice, a rectangle of linen tied about the shoulders, was originally a neck cloth to protect the vestments and is now optional in most places. The cincture, a cord at the waist, gathers it and has its own prayer about chastity in the older vesting formulae.
Two derivatives are more familiar to most churchgoers than the alb itself. The surplice is a wide sleeved white garment worn over a cassock for the office and for non Eucharistic services, and the cotta is a shorter version of it. Choirs and servers generally wear one of these rather than an alb.
The alb's indifference to rank is the point worth holding on to. Everything else in the vestry says who someone is. The alb says only that they are baptised, which is the condition of being in the sanctuary at all.