Lamarck explained evolution by putting forward or promulgating two ideas:
-The use and disuse of body part
-The inheritance of acquired trait
Lamarck had observed that a particular part of the body that is constantly put into use enlarges and becomes more efficient,  while the body part is not constantly used becomes weak, small and deteriorate, through this type of use and disuse, an organism acquires some traits in its lifetime. Lamarck opined that these acquired traits can be passed on to offspring; hence, he believed that new species evolved after  several generations, by acquiring new traits and losing old ones.
He cited the example of giraffes whose ancestors had short necks. They kept stretching their necks to reach for the leaves of the trees as a result of this constant stretching, it made their nec to become slightly longer. The acquired trait was passed on to offspring and then to subsequent generations. Thus at each successive generation their neck kept becoming longer, until the present day giraffes.
Lamarck was right when he stated that acquired traits could be from use and disuse of body parts, for instance, muscular power and accumulation of muscles could result through continued exercise. However he was wrong by concluding that acquired traits are inheritable because traits that are inheritable are genetic rather than solely phenotypic.
Briefly summarized, his theory states the following:
-The environment  introduced a need for some structure in the organism
-The organism attempt to meet this need
-In response to its efforts the structure of the organism is passed on to its offspring
-The classic example used to explain this theory is the development of long neck in giraffe, but this is untenable/unacceptable in view of our