The literary term "expressionistic" is an adjective meaning "having characteristics similar to those found in the literary and artistic movement known as Expressionism". It should not be confused with such related terms as "expressive", which have more general meanings outside of literary studies. Expressionism was an international movement in the arts and literature in the late nineteenth and early (pre-World War I) twentieth century. The Expressionists in art were known for bold dramatic colors and compositions that attempted to mirror inner states as much as accurately representing external realities. Leading expressionist artists included Paul Gaughin and Vincent van Gogh. In literature, Expressionism was also concerned with expressing internal states, often of people in extreme moments of stress in which inner, symbolic or dream states were revealed. A leading expressionist dramatis was August Strindberg.
In simple terms, perhaps, we could define 'expressionism' as an Artistic/Literary Theory, which developed aound late 19th and early 20th centuries.
According to this, the inner or 'subconscious' thoughts, feelings/emotions etc, are depicted via a number of writing techniques that are 'non-naturalistic' (please see 'Naturalism' too)-- such as in a symbolic manner, or in a fantasy/dreamlike state etc.
I hope thats clearer?