Create a query that joins the two tables. In addition to the fields from the main table, select the description column from the lookup table. Then use that query as the recordsource for the form. Make sure that you set the Enabled property of the description field to No so that it will not be updateable on this form. This is called an "auto lookup" query. You can read about the concept in help. If you change the value of the code field on the form, Access will automatically change the value of the description field.
To make updating easier, it is common to use a combobox control for the code field. Set the limit to list property to Yes. This will ensure that only valid values can be entered in the code field.
You can see an example of this technique in the Orders.mdb and also in Northwind.mdb.