SAP Search helps or matchcodes as they were referred to in the early days provide the F4 field entry help for a user. I.e. when a user is inputting data into a field and they use the search functionality to find the correct value they want to enter they are using a search help.
These can be implemented in many different ways:
Attached to a Table or Table field
Attaching at the database level is probably the most common and once it is attached any report, program or screen field that refers to this database object will automatically contain the search help functionality. For search helps that have many exporting parameters this method allows you to link the search help to many fields of a table or database structure so that multiple input fields can be populated from a single user search. Implementation is also very easy using this method, via SE11 you simple use menu option "Goto->Search help->for field" to add to a table field and "Goto->Search help->for table" to add to the table
Attached to data element
You can even assign search help at the data element level in much the same way but you do not have as many options when doing it this way. You simply go into edit mode and add the name and parameter. Once added any dictionary field that references it will include the assigned search help. Please note the search help attached at field or table level will take priority of one attached at DE level.
Attached at screen level
When developing dialog screen transactions you have the option to assign a search help within the attributes of the screen field. This again has limited functionality and can only be used to populate a single input field.
Attached to an ABAP report selection screen parameter
Using the MATCHCODE ABAP statement you can also attached a matchcode to your selection screen parameters. A matchcode is simply a search help, in the early days match codes had to have a name that was 5 characters or less in length. This is no longer the case and any search help can be used.
PARAMETERS: P_param TYPE c MATCHCODE OBJECT ZSHELP.
Index list of available SAP search helps