Martin Heidegger, the 20th-century German philosopher, introduced to the world a large body of work which intended a profound change of direction for philosophy. Such was the depth of change that he found himself needing to introduce a number of neologisms and adapted vocabulary, often connected to idiomatic words and phrases in the German language.
Two of his most basic neologisms, present-at-hand and ready-to-hand, are used to describe various attitudes toward things in the world. For Heidegger, such "attitudes" are prior to, i.e. more basic than, the various sciences of the individual items in the world. Science itself is an attitude, one that attempts a kind of neutral investigation. Other related terms are also explained below.
Heidegger's overall analysis is quite involved, taking in a lot of the history of philosophy. See Being and Time for a description of his overall project, and to give some context to these technical terms.