Players gather resources to build units and defeat their opponents. To that end, they often have worker units (and extraction structures) that can gather resources needed to build workers, buildings, other units and research upgrades.
Workers are often weak in fights compared to other units.
Buldings and research define technology trees (directed acyclic graphs) and each state of a tech tree allow for the production of different unit types and the training of new unit abilities.
An opening denotes the same thing as in Chess: an early game plan for which the player has to make choices.
That is the case in Chess because one can move only one piece at a time (each turn), and in StarCraft because, during the early game phase, one is economically limited and has to choose which tech paths to pursue.
Available resources constrain the technology advancement and the number of units one can produce. As producing buildings and units also take time, the arbitrage between investing in the economy, in technological advancements, and in units production is the crux of the strategy during the whole game.
In StarCraft an opening refer to the initial moves of a game. The term can refer to the initial moves by either side, but an opening by Zerg may also be known as defense. There are dozens of different openings, and hundreds of variants. These vary widely in character from quiet positional play to wild tactical play.
In addition to referring to specific move sequences, the opening is the first phase of a game, the other phases being the middlegame and the endgame.
Opening moves that are considered standard are referred to as "book moves". Reference works often present move sequnces in simple algebraic notation, opening trees or theory tables. When a game begins to deviate deom known, opening theory, the players are said to be "out of the book".
Professional players spend yers studying openings, and continue doing so throughout their careers, as opening theory continues to evolve.
The study of openings can become unbalanced if it is to the exclusion of tactical training and middlegame and endgame strategy.