Each of the member churches of the Anglican Communion has some process by which it governs itself. In the United States, the Episcopal Church holds a General Convention every 3 years, at which the canons of the church are updated.
A General Synod is the same kind of event as a General Convention, but in different countries. For example, England, Canada, Australia, and New Zealand hold periodic General Synods as part of their church governance process.
The term "holy orders" is a way of referring to ordination: an ordained person such as a priest or deacon is spoken of as "being in holy orders," meaning that the person has made priestly vows, received the laying-on of hands, and has been admitted (by a bishop) into one of the levels of ordination.