Buy and hold investment strategies are exactly that. Rather than trading regularly securities are purchased and held for a long period of time (usually many years).
Buy and hold strategies are not necessarily passive or mechanical, although many are. A buy and hold investor may actively select investments, but hold investments once bought, and review a portfolio with a view to buying and selling relatively infrequently.
The thinking behind a purely passive buy and hold strategy is that of other passive strategies, with, perhaps, a greater emphasis on minimising costs — unlike passive strategies such as index tracking that require constant re-balancing.
More active buy and hold strategies usually rely on the assumption that it is possible to select mispriced securities (usually shares), implying that markets are not efficient, but that these mispricings will be corrected in the long term.