An objective function is a measure of how similar a prediction of a value and the actual value are. Usually, we are looking to find the set of parameters that lead to the smallest possible cost which would imply that your algorithm will perform well.
Tutorial, Learning Analytics Summer Institute, Ann Arbor, June 2017 As algorithms pervade societal life, they’re moving from an arcane topic reserved for computer scientists and mathematicians, to the object of far wider academic and mainstream media attention
K. Fleszar, M. Mnich, and J. Spoerhase. 24th Europ. Symp. Algorithms (ESA'16), volume 57 of Leibniz International Proceedings in Informatics (LIPIcs), page 42:1--42:17. Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik, (2016)