In order to access Family Tree or the Photos option from the FamilySearch home page, you need to sign in to FamilySearch. If you have an existing FamilySearch account, you can sign in using your username and password. If you do not have an existing account, you can easily create one by clicking the Sign In option from the FamilySearch home page, and then click Create an Account. Once you complete the registration, you will have an account you can use to sign in to FamilySearch.
If you sign in to FamilySearch using the default settings, you can stay signed in for up to 24 hours. If during that 24 hours you are not active on the site for 1 continuous hour, FamilySearch will automatically sign you out of the site. Once FamilySearch has signed you out of the site, you will need to sign in again with your username and password to access the Family Tree or Photos options. This is the established way FamilySearch handles no activity on the site.
What is the signed in for 2 weeks check box just above the sign in button?
If you check the box to stay signed in for 2 weeks and sign in to FamilySearch, you will not be signed out of the site after one continuous hour of inactivity. This feature can be very helpful if you are researching documents or other websites that take you away from activity on FamilySearch for more than 1 hour. It is recommended that you do not use this option on a public or shared computer.
After you have been signed in for two weeks, FamilySearch will automatically sign you out. If you sign in to FamilySearch with this option checked, it will renew this option so you can continue to use the signed in for 2 weeks feature.