The phrase "observer missions" refers to a group of people or individuals who are sent to watch and monitor a situation or event. They do not actively participate, but instead observe and report what they see. Full definition
Although there is no direct relation between election observation and successful elections an independent observer mission has a number of benefits. (blog.politics.ox.ac.uk)
Response 20: The Chairperson's managerial competence is evident from her career track record and the reports of local and international election observer missions as well as the commendations from regional directors and senior staff of the EC on the organization of the 2016 elections. (ghanapoliticsonline.com)
The various observer missions monitoring the elections in Kenya have called for peace in that country. (citifmonline.com)