Open adoption is a term used to describe the contact Birth Parents have with the baby after adoption.
All adoptions used to be Closed. Adoption records and original birth certificates were "sealed". Adoptive parents and Birth parents did not meet and Birth parents did not know anything about the baby after the adoption. The baby and later the adult, could not access the records.
Now many adoptions are Open or Semi-Open. Neither term has an exact definition.
An adoption may be "totally open." This could include visits, shared pictures and letters, emails or whatever communication is desired. They might be celebrations of some birthdays or holidays. Sometimes Birth parents and Adoptive parents become like family to each other.
Semi-open adoption can mean letters and pictures would be shared at agreed-upon intervals. More rarely, a visit could take place. And there are lots of arrangements somewhere in between.
In a closed-type adoption people aren't really in contact.
The thing to keep in mind in modern adoption is that it is really up to the Birth parents and the Adoptive parents to decide what they want together and work that out. Some people will change their relationship over time, becoming more open if everyone is comfortable. Sometimes what Birth parents want changes over time.
I heard a Birth mom speak at an adoption seminar Don and went to. In her adoption plan it was agreed that the Adoptive parents would send pictures and letters 4 times the first year and 2 times per year after that until the child was 18 and then the child could decide if he wanted to meet her. She did not want a whole lot of contact because she thought it might make her sad. She was satisfied with this arrangement.
People can basically make any arrangement they want to as long as everyone agrees to it. People will often write up the agreement and sign it as part of the adoption paperwork. If they want to be more open over time they can, but not less. That is, if the Adoptive parents agreed to send letters and pictures 3 times per year then they are expected to keep that going.