What is SLIS in sap abap ALV

SLIS is the type library for ALV grid.

If you'll use the ALV you have to add TYPE-POOLS : SLIS. command at the beginning of your code.

Consider these :
slis_t_fieldcat_alv is containing "_t_"
It means that it is an internal table and slis_fieldcat_alv is header line of that.

Here is a practical example for alv grid :
Just think that you have an internal table named 'ITAB' to show.

Step1 : First add these lines to your code :
TYPE-POOLS : SLIS.

DATA ALV_PROG_NAME LIKE SY-REPID.
ALV_PROG_NAME = SY-REPID.

DATA : ALV_ITAB_NAME(30),
L_FIELDCAT TYPE SLIS_T_FIELDCAT_ALV.
ALV_ITAB_NAME = 'ITAB'. "!!Write here the name of your internal table

Step 2 : Add these two function :
The first function is filling the fieldcat L_FIELDCAT that you described, second is showing it on the screen.

CALL FUNCTION 'REUSE_ALV_FIELDCATALOG_MERGE'
EXPORTING
I_PROGRAM_NAME = ALV_PROG_NAME
I_INTERNAL_TABNAME = ALV_ITAB_NAME
* I_STRUCTURE_NAME =
* I_CLIENT_NEVER_DISPLAY = 'X'
I_INCLNAME = ALV_PROG_NAME
* I_BYPASSING_BUFFER =
* I_BUFFER_ACTIVE =
CHANGING
CT_FIELDCAT = L_FIELDCAT
EXCEPTIONS
INCONSISTENT_INTERFACE = 1
PROGRAM_ERROR = 2
OTHERS = 3
.
IF SY-SUBRC <> 0.
* MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO
* WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.
ENDIF.

CALL FUNCTION 'REUSE_ALV_GRID_DISPLAY'
EXPORTING
* I_INTERFACE_CHECK = ' '
* I_BYPASSING_BUFFER =
* I_BUFFER_ACTIVE = ' '
I_CALLBACK_PROGRAM = ALV_PROG_NAME
* I_CALLBACK_PF_STATUS_SET = ' '
* I_CALLBACK_USER_COMMAND = ' '
* I_CALLBACK_TOP_OF_PAGE = ' '
* I_CALLBACK_HTML_TOP_OF_PAGE = ' '
* I_CALLBACK_HTML_END_OF_LIST = ' '
* I_STRUCTURE_NAME =
* I_BACKGROUND_ID = ' '
* I_GRID_TITLE =
* I_GRID_SETTINGS =
* IS_LAYOUT =
IT_FIELDCAT = L_FIELDCAT
* IT_EXCLUDING =
* IT_SPECIAL_GROUPS =
* IT_SORT =
* IT_FILTER =
* IS_SEL_HIDE =
* I_DEFAULT = 'X'
* I_SAVE = ' '
* IS_VARIANT =
* IT_EVENTS =
* IT_EVENT_EXIT =
* IS_PRINT =
* IS_REPREP_ID =
* I_SCREEN_START_COLUMN = 0
* I_SCREEN_START_LINE = 0
* I_SCREEN_END_COLUMN = 0
* I_SCREEN_END_LINE = 0
* IT_ALV_GRAPHICS =
* IT_ADD_FIELDCAT =
* IT_HYPERLINK =
* I_HTML_HEIGHT_TOP =
* I_HTML_HEIGHT_END =
* IT_EXCEPT_QINFO =
* IMPORTING
* E_EXIT_CAUSED_BY_CALLER =
* ES_EXIT_CAUSED_BY_USER =
TABLES
T_OUTTAB = ITAB[] "Write here the name of your internal table + []
* EXCEPTIONS
* PROGRAM_ERROR = 1
* OTHERS = 2
.
IF SY-SUBRC <> 0.
* MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO
* WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.
ENDIF. *-- Bulent